NAME

std::partition_point - std::partition_point

Synopsis

Defined in header <algorithm>
template< class ForwardIt, class UnaryPredicate > (since C++11)
ForwardIt partition_point( ForwardIt first, ForwardIt last, (until C++20)
UnaryPredicate p );
template< class ForwardIt, class UnaryPredicate >
constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, (since C++20)
UnaryPredicate p );

Examines the partitioned (as if by std::partition) range [first, last) and locates
the end of the first partition, that is, the first element that does not satisfy p
or last if all elements satisfy p.

Parameters

first, last - the partitioned range of elements to examine
unary predicate which returns true for the elements found in the
beginning of the range.

The expression p(v) must be convertible to bool for every argument v
p - of type (possibly const) VT, where VT is the value type of ForwardIt,
regardless of value category, and must not modify v. Thus, a parameter
type of VT&is not allowed
, nor is VT unless for VT a move is equivalent to a copy
(since C++11).

Type requirements

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPredicate must meet the requirements of Predicate.

Return value

The iterator past the end of the first partition within [first, last) or last if all
elements satisfy p.

Complexity

Given N = std::distance(first, last), performs O(log N) applications of the
predicate p.

However, for non-LegacyRandomAccessIterators, the number of iterator increments is
O(N).

Notes

This algorithm is a more general form of std::lower_bound, which can be expressed in
terms of std::partition_point with the predicate [&](auto const& e) { return e <
value; });.

Example

// Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>

auto print_seq = [](auto rem, auto first, auto last) {
for (std::cout << rem; first != last; std::cout << *first++ << ' ') {}
std::cout << '\n';
};

int main()
{
std::array v = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

auto is_even = [](int i){ return i % 2 == 0; };

std::partition(v.begin(), v.end(), is_even);
print_seq("After partitioning, v: ", v.cbegin(), v.cend());

const auto pp = std::partition_point(v.cbegin(), v.cend(), is_even);
const auto i = std::distance(v.cbegin(), pp);
std::cout << "Partition point is at " << i << "; v[" << i << "] = " << *pp << '\n';

print_seq("First partition (all even elements): ", v.cbegin(), pp);
print_seq("Second partition (all odd elements): ", pp, v.cend());
}

Possible output:

After partitioning, v: 8 2 6 4 5 3 7 1 9
Partition point is at 4; v[4] = 5
First partition (all even elements): 8 2 6 4
Second partition (all odd elements): 5 3 7 1 9